1. Field of the Invention
The invention relates to a method for acquisition, reduction and transmission of Earth observation satellite images, and more particularly to a method for acquisition, reduction and transmission of Earth observation satellite images employed on board a satellite, particularly in low orbit.
2. Description of the Related Art
An Earth observation satellite, in particular a satellite in low orbit, intended to take systematic images of the globe is capable of acquiring a large number and variety of images in a relatively short time interval. This variety of images results in particular from the fact that these satellites fly over various regions of the globe during their orbits, inter alia oceans, deserts, cities, forests, polar regions, etc. There are instruments fitted on board satellites which are intended to acquire only one particular type of region, for example the MERIS instrument fitted on the ENVISAT satellite, which is intended to measure the color of the oceans. It is known to provide the satellites with means for determining the periods during which each instrument for acquisition of data, in particular images, carried by the satellite does not have to acquire images because the overflown regions do not correspond to the intended objective of this instrument. This furthermore makes it possible to limit the bandwidth and not to use the resources needlessly. For each instrument, these non-observation periods are generally defined by the coordinates of the regions not to be observed. These excluded regions may, depending on the intended objective of the instrument, be oceans, forests, cities, poles, etc. Furthermore, for instruments which operate in the visible spectrum, these excluded regions may also comprise regions in which it is night-time. This is because during a part of its orbit, a satellite flies over regions in which it is night-time and for which the acquisition of images is not of interest.
Therefore, in the known methods:                means for identifying the geographical regions flown over determine whether acquisition of an image of an overflown region is to be carried out,        acquisition of an image of this region is carried out if the identification means have determined that an image of this overflown region should be acquired,        this image is compressed by image compression means,        this compressed image is transmitted to a remote image reception device.        
The means for identifying the geographical regions flown over make it possible to exclude a certain number of geographical regions of the globe. This technique of excluding certain geographical regions of the globe forms an integral part of the strategy of limiting the bandwidth necessary for transmitting the data acquired by an instrument to the ground. This limitation of the bandwidth is also obtained by compressing the images actually acquired, before they are transferred to the ground. This compression of the images may be obtained by a wide variety of compression algorithms, inter alia algorithms of the JPEG, JPEG2000 type, etc.
The Inventors have determined that the drawback of these known methods resides on the one hand in the fact that certain regions of the globe are totally ignored even though they may be of interest, and on the other hand in the fact that the compression algorithm is not adapted to the acquired images. This is the case for example with oceans, which are generally removed from protocols for observing regions of human activity, even though images of these regions could reveal relevant and useful data, such as boats, oil platforms, etc.
Furthermore, definition of the regions to be excluded is a relatively complex task, in particular for satellites in low orbit given that it involves the orbit of the satellite, the rotation of the Earth, etc. Furthermore, unless meteorological data are communicated constantly to the satellite, the meteorological conditions will not be known to the satellite so that the instrument is liable to acquire and transmit cloudy images which are of no interest.
Furthermore, the quality of the compression of an image provided by the known compression methods depends on the type of image to be compressed. In particular, the Inventors have determined that an algorithm which is effective at compressing an image of a large city is not suited to compressing an image of the tropical forest.
U.S. Pat. No. 6,031,939 describes a method for optimizing the compression of image data with automatic selection of the compression conditions. In this method, a plurality of compression algorithms are evaluated in relation to the images to be compressed, or test images, and the algorithm to be used is determined automatically on the basis of parameters representative of the objective of the user, selected by him, and after evaluation for each algorithm. Various algorithms may be applied to different portions of the same image. Likewise, the publication “An adaptive technique to maximize lossless image data compression of satellite images” XP004011783 describes techniques for lossless compression of satellite images, which are based only on the analysis of technical parameters (entropy, texture, regularity of the patterns, etc.) of the images. Different coding techniques are applied as a function of the different entropies of the various regions of the image. The compression performance of these methods remains limited by the fact that only a purely technical analysis of the image (luminosity, texture, contrast, etc.) is carried out.